Curiosities of Science. 97 



the most useful combination of metals for specula, both in whiteness, 

 porosity, and hardness, to be copper and tin. Of this compound the re- 

 flector was cast in pieces, which were fixed on a bed of zinc and copper, 

 a species of brass which expanded in the same degree by heat as the 

 pieces of the speculum themselves. They were ground as one body to 

 a true surface, and then polished by machinery moved by a steam- 

 engine. The peculiarities of this mechanism were entirely Lord Rosse's 

 invention, and the result of close calculation and observation : they were 

 chiefly, placing the speculum with the face upward, regulating the tem- 

 perature by having it immersed in water, usually at 55 Fahr,, and re- 

 gulating the pressure and velocity. This was found to work a perfect 

 spherical figure in large surfaces with a degree of precision unattainable 

 by the hand ; the polisher, by working above and upon the face of the 

 speculum, being enabled to examine the operation as it proceeded with- 

 out removing the speculum, which, when a ton weight, is no easy matter. 

 The contrivance for doing this is very beautiful. The machine is 

 placed in a room at the bottom of a high tower, in the successive floors 

 of which trap-doors can be opened. A mast is elevated on the top of the 

 tower, so that its summit is about ninety feet above the speculum. A 

 dial-plate is attached to the top of the mast, and a small plane speculum 

 and eye-piece, with proper adjustments, are so placed that the combin- 

 ation becomes a Newtonian telescope, and the dial-plate the object. 

 The last and most important part of the process of working the specu- 

 lum, is to give it a true parabolic figure, that is, such a figure that each 

 portion of it should reflect the incident ray to the same focus. Lord 

 Eosse's operations for this purpose consist 1st, of a stroke of the first 

 eccentric, which carries the polisher along one-third of the diameter of 

 the speculum ; 2d, a transverse stroke twenty-one times slower, and 

 equal to 0'27 of the same diameter, measured on the edge of the tank, 

 or 1'7 beyond the centre of the polisher ; 3d, a rotation of the speculum 

 performed in the same time as thirty-seven of the first strokes ; and 

 4th, a rotation of the polisher in the same direction about sixteen times 

 slower. If these rules are attended to, the machine will give the true 

 parabolic figure to the speculum, whether it be six inches or three feet 

 in diameter In the three-feet speculum, the figure is so true with the 

 whole aperture, that it is thrown out of focus by a motion of less than 

 the thirti-.th of an inch, "and even with a single lens of one-eighth of 

 an inch focus, giving a power of 2592, the dots on a watch-dial are still 

 in some degree defined." 



Thus was executed the three-feet speculum for the twenty- 

 six-feet telescope placed upon the lawn at Parsonstown, which, 

 in 1840, showed with powers up to 1000 and even 1600 ; and 

 which resolved nebulae into stars, and destroyed that symmetry 

 of form in globular nebulae upon which was founded the hypo- 

 thesis of the gradual condensation of nebulous matter into suns 

 and planets.* 



Scarcely was this instrument out of Lord Rosse's hands, 

 when he resolved to attempt by the same processes to construct 



* This instrument also discovered a multitude of new objects in the moon; 

 as a mountainous tract near Ptolemy, every rid^e of which is dotted with ex- 

 tremely minute craters, and two black parallel stripes in the bottom of Ar>star- 

 chus. Dr. Robinson, in his address to the British Association in 1843, stated that 

 in this telescope a building the size of the Court-house at Cork would be easily 

 visible on the lunar surface. 



